This invention relates to an amplifier having first and second input terminals and an output terminal, the first input terminal being connected to a fluid level sensor applying to the terminal at least two distinctive different levels of electrical potential, the second input terminal being connected to a reference potential and the output terminal being connected to a switch device. One of the distinctive levels, having greater magnitude than the reference potential, operates the switch device to take a first position and the other of the distinctive levels, having lower magnitude than the reference potential, operates the switch device to take a second position. The first and second positions correspond to connecting and disconnecting of a load to a power supply.
A known amplifier of the kind disclosed above is intended to cooperate with sensors of the thermistor type, particularly those having a positive temperature coefficient. For example, such a PTC-thermistor at temperatures between 20.degree. C. and about 150.degree. C. has a resistance of about 100 ohms, whereas the thermistor at higher temperatures has a resistance of the order of several kiloohms. The change of resistance occurs very abruptly at the resistance switching temperature, in the example 160.degree. C. The sensor is positioned in a fluid container or tank to indicate during filling of the tank when the fluid has reached a predetermined level. An electric current of suitable magnitude flows through the thermistor to increase the temperature of the thermistor, when surrounded by air, above the resistance switching temperature. The thermistor then assumes its high resistance condition. The predetermined fluid level is indicated when the thermistor has changed its resistance into the low resistance condition. This is caused by the cooling action of the fluid when surrounding the thermistor.
It is desirable that the amplifier disclosed above can alternatively be used together with sensors of the switch type, i.e. sensors that have a connection between two terminals that is either open or closed. Further, it is desirable that the amplifier when adapted to switch type sensors cannot be responsive to a thermistor sensor to operate the switch device to its second position in order to connect the load to the power supply.
It is an object of the invention to meet the requirements indicated above. To achieve this object, the invention is characterized by the features stated in the appending claims.
Briefly stated, in accordance with the invention, the above objects are achieved by providing an operational amplifier having a reference voltage connected to one input. When an input sensor is in the form of a switch, the switch is connected to the other input terminal of the amplifier by way of an interface responsive to the open or closed state of the switch for applying a voltage above or below the reference voltage level to the other input of the amplifier.
In one advantageous system, in accordance with the invention, the output of the amplifier may comprise a switching circuit. If the amplifier system of the invention is employed in a level sensing system, the output circuit may be connected to operate a pump, enabling filling of a tank in which the sensor is located.
In a further advantageous embodiment of the invention, the interface comprises a transistor circuit having two input terminals and an output terminal whereby the switch sensor is connected across the two input terminals and the other input of the amplifier is connected to the output terminal of the interface. The interface may comprise a transistor circuit responsive to the state of the switch for controlling the voltage applied to the other amplifier input.
In accordance with a still further embodiment of the invention, a two-way switch may be provided in the interface to enable the alternate use of a thermistor in place of the sensor switch, the thermistor being connected to the same terminals as the sensor switch had been. In this case, the interface is provided with a switch disabling use of the interface. When the sensor is connected in the circuit, voltage dividers at the input of the amplifier apply a voltage to the thermistor such that it is heated when not immersed in the liquid in the tank.